Systems for dehumidification or drying of moist gas streams are known since long. Examples of such systems are found in, among others, U.S. Pat. No. 4,701,189 and US 2005/0235827, which disclose rotary drying wheels. The drying wheel contains a mass of absorbing or sorptive material (drying agent) that removes moisture from the gas stream by the vapour being absorbed. The moisture is picked up in the drying agent and the dry gas stream is forwarded for the use in a process or the like. After a prolonged dehumidification, the drying agent will be saturated with moisture, and therefore it is required that the moisture has to be driven out of the drying agent for enabling the drying to proceed. This is usually made by regeneration, i.e., that a heated gas stream passes through the drying wheel in an area separated from the passage of the moist gas stream. The heated air regenerates the drying wheel continuously, which thereby gives a continuous drying of the moist gas stream.
A problem of such drying wheels is that, upon absorption of vapour, latent heat in the gas stream is converted into sensible heat, i.e., the evaporation energy in absorption increases the temperature of the gas and drying agent, which impairs its absorption capacity. Likewise, also the regeneration increases the temperature of the drying wheel.
US 2011/0132191 and US 2012/0125198 disclose proposals of solutions wherein a separate gas stream for the cooling of the absorbing drying agent flows through the drying wheel in a delimited zone or sector separated from the main flow of the gas to be dried and the regeneration flow. Disadvantages of these solutions are that they require an additional flow for the cooling gas with associated equipment and that the available sector for the drying of the gas in the main flow is reduced, which impairs the capacity of the drying wheel. Another disadvantage is that the cooling sector only cools the drying agent in the beginning of its revolution; during the remaining part of the revolution, the drying agent is heated during the absorption.
JP 60-102919 discloses a system for dehumidification of air with high air humidity using a dehumidifying rotor, wherein a part of a dehumidified air stream is passed through a heat exchanger rotor that lowers its temperature, whereupon a part of this dehumidified cooled air is mixed with moist air in order to lower the air humidity of the incoming air that then is sent to the dehumidifying rotor. A disadvantage of this system is that the cooled air does not essentially contribute to the lowering of the temperature of the rotor. Therefore, there is a need of developing systems and methods for improving the performance of sorption dryers, particularly for high air humidities.